The present invention relates to an image forming apparatus to conduct an image formation in an electrophotographic method with an arrangement in which an image exposing means is provided in an photoreceptor drum and a charging means and a developing means are provided on the external periphery of the photoreceptor drum, in a copying machine, a printer, or a facsimile machine.
As a method of forming a multi-color image, the following color image forming apparatus are known: (1) there are provided photoreceptor drums, charging devices and developing devices whose number are identical to that of colors necessary for a color image formation and a color image is formed by transferring a monochromatic toner image from a photoreceptor drum to a recording sheet by a transfer device and by superimposing plural monochromatic toner images on the recording sheet, (2) a photoreceptor drum is rotated plural times so that charging, image exposure and developing for each color are repeated for forming a full color image, and (3) charging, image exposure and developing are successively conducted during one rotation of the photoreceptor drum for forming a color image.
However, the color image forming apparatus (1) has drawback to become a large size in volume since it is necessary to provide plural photoreceptor drums and a device to convey recording sheets to the drums. On the other hand, the color image forming apparatus (2) has a merit that a volume of the apparatus can be made small. However, a size of the image is limited to a size smaller than the surface of the photoreceptor drum.
The color image forming apparatus (3) has merits that there is no limitation in the image size and that an image can be formed at a high speed. However, since plural charging devices, plural image exposing devices and plural developing devices are provided around the external periphery of the photoreceptor drum, a size of the apparatus becomes large. To make a size of an apparatus smaller, Japanese patent unexamined publication (Tokukaihei 5-307307) discloses an image forming apparatus (4) in which a transparent drum is used as a base drum of a photoreceptor drum and image exposing means are provided in the photoreceptor drum.
In the apparatus (4), since plural image exposing devices are provided inside the photoreceptor drum, a mounting structure of the photoreceptor drum becomes complicate. Further, lead wires are connected for exposure elements of the exposing devices and the lead wires are needed to be drawn out of the photoreceptor drum and construction members to mount the photoreceptor drum. In the time of mounting, dismounting or replacing the photoreceptor drum, the lead wires is brought in contact with the photoreceptor drum or the construction members and the weight of the drum or the members are loaded on the lead wires. As a result, the lead wires are broken or the image exposing elements are deformed by being pulled with the lead wires. The deformed image exposing elements causes deviation of image light or out of image focus.
Further, on supporting members on which the exposing devices are mounted, there are provided a transfer-simultaneous exposing means for conducting uniform exposure simultaneously with transferring operation, a light detecting means for conducting toner concentration correction and .gamma.-correction and a memory member for memorizing positional registration information in the time of incorporating the exposure devices in the photoreceptor drum and information as to photoreceptor characteristic. In addition to the lead wires for the exposing devices, lead wires connected to the transfer-simultaneous exposing means, the light detecting means and the memory member. In the time of mounting, dismounting or replacing the photoreceptor drum, the lead wires is brought in contact with the photoreceptor drum or the construction members and the weight of the drum or the members are loaded on the lead wires. As a result, the lead wires may be broken or the transfer-simultaneous exposing means, the light detecting means and the memory member may be shifted by being pulled with the lead wires or may be deformed. The transfer-simultaneous exposing means, the light detecting means and the memory member may not function normally.
Further, through a window provided on constructing members to mount the photoreceptor drum in order to draw the lead wires to the outside, scattering toner and dust come inside the image forming apparatus and soil the image exposing means and the light detecting means, resulting in malfunction of the exposing means and the light detecting means.
As another problem in the above apparatus (4), when replacing the photoreceptor drum with a spare one due to the reason that a usable time period of the photoreceptor is ended when a predetermined number of copy sheets are used, correction for positional registration between the photoreceptor drum and the image exposing means is needed. Further, in the time of initially setting an image forming apparatus or in the time of replacing the photoreceptor drum, correction for photoreceptor drum characteristic and correction for exposure amount of image exposing means are needed. The correction becomes troublesome and complicate.